Field of the Invention
The invention, according to the various embodiments described herein, relates to a power measuring device which uses diode detectors.
Discussion of the Background
Sensor heads for high-frequency (HF) power measurement operate either thermally or use diode detectors, for example, on the basis of Zero-Bias Schottky diodes or diodes using ‘Planar Doped Barrier Technology’. Diodes for high-frequency power rectification are optimised for a favourable HF behaviour, for example, a low barrier-junction capacitance. However, this is achieved at the cost of robustness. These highly sensitive components can be irreversibly damaged even by very brief overshooting of the maximal power or by electrostatic discharges. This is not always evident. A total breakdown need not always be present. In many cases, a plausible measurement result can still be displayed, even if the permitted measurement deviations have already been exceeded.
For this reason, it is conventional to test or to calibrate HF power-sensor heads regularly with reference to an external calibration source. However, this method is disadvantageous, because the sensor head must be connected manually to the calibration source. In particular, in the case of a test setup in the factory, the sensor head would have to be removed from this setup. This is not desirable for a smooth manufacturing procedure. Added to this, with the use of USB power-sensor heads, such external calibration sources are not reliably available. For implementation of the measurement with such a sensor head, only a conventional PC is required.
Accordingly, a power-measuring device with internal calibration is available. In this context, a current-voltage characteristic of the detector diodes is measured. For this purpose, a known reference voltage of variable level is applied to the detector diode by means of a measurement amplifier, and the resulting current is measured. However, this testing of the functionality of the detector diode functions only if the measurement amplifier is operated in an inverted configuration, that is, if it provides a low-ohmic input. This is frequently the case with broadband sensor heads, where the open-output voltage is not measured, but instead, the short-output current of the diode detector in order to realise the short decay times in the range of a few nanoseconds required by the application. Broadband sensor heads are preferably used in a manner comparable with an oscilloscope to measure the envelope curve, that is, the time-variable amplitude of pulsed and/or modulated high-frequency measurement signals. However, in the case of sensor heads for the measurement of mean power which drive the detector in the quadratic range of the rectifier diodes, the measurement amplifier has a non-inverting configuration. This is necessary in order to measure the transformed measurement signal, the open-output voltage of the diode detector, as far as possible without falsification. The calibration cannot therefore be used with sensor heads of this kind. Furthermore, with the calibration shown in this context, the rectification effect of the detector in the quadratic range cannot be judged with a sufficiently high level of accuracy.